1. Field of the Invention
The present invention related to a thin film transistor, particularly to a thin film transistor with UV light absorber layer.
2. Description of the Prior Art
As for common metal oxide semiconductors based thin film transistor, the active layer made of In—Ga—Zn—O, electrodes, gate insulator and protection layer are used to form the thin film transistor.
FIG. 1 illustrates the cross-sectional view of the conventional thin film transistor. The conventional thin film transistor 100 is mostly applied in the displays and relevant IC products, such as the active matrix thin film transistor liquid crystal display (AMTFT LCD) and active matrix organic light emitting diodes (AMOLED). Because it is necessary to reach the threshold voltage of active layer to activate the transistor for outputting the current of source electrode, thus the threshold voltage will affect the efficiency of current output, and further affect the performance of applied display, OLED or IC. Common metal oxide semiconductor material possesses the range of visible light transmittance. However, the valance level in the energy gap of active layer often has the energy gap defect of carrier capture. The energy gap defect will generate the electron-hole coupling by the activation of energy (such as photon), and then further affect the threshold voltage of active layer.
As for the active matrix thin film transistor liquid crystal display (AMTFT LCD) and active matrix organic light emitting diodes (AMOLED), the repeated arrangement of pixel and the input of signal are adopted to reach the image display function of displays in the operation process of thin film transistor displays. However, the thin film transistors (TFTs) are used to control the liquid crystal capacitance outputted to every pixel by the voltage. Also, TFTs are used to control the driving current of OLED which means TFTs dominate the lightness of OLED. Under the illumination of back light or external light, common metal oxide thin film transistor will change the transform characteristic curve due to the electron-hole coupling by the activation of light, and then change the threshold voltage, as shown in FIG. 2.
FIG. 2 illustrates the variation of transform characteristic curve for the threshold voltage under light illumination. As shown in FIG. 2, because there is more metal oxide semiconductor material, there is stronger photosensitive for the light in the range of ultraviolet light, thus there is severe variation of threshold voltage when the illumination light wavelength is smaller than 460 nm. When the variation of threshold voltage is larger, the instability of thin film transistor will be increased.